Method and apparatus for energy recovery in an environmental control system

ABSTRACT

Control apparatus for an environmental control system comprises input circuitry receiving environmental information and output circuitry for controlling an HVAC system. Processing circuitry in the controller configures the output circuitry based at least in part on the signals received on the input circuitry. Information about the status of the HVAC system may be transferred to system administrators using a wireless link.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of, and claims priority to, U.S.Patent application Ser. No. 09/746,213, filed on Dec. 22, 2000, now U.S.Pat. No. ______; which application in turn is a continuation of andclaims priority to U.S. patent application Ser. No. 09/351,974, filed onJul. 12, 1999, now U.S. Pat. No. 6,176,436 issued on Jan. 23, 2001;which application in turn is a continuation of, and claims priority toU.S. patent application Ser. No. 08/933,871, filed on Sep. 19, 1997,U.S. Pat. No. 6,062,482 issued on May 16, 2000. The content of U.S.patent application Ser. Nos. 09/746,213, 09/351,974 and 08/933,871 arehereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to controllers for heating,ventilation, and air conditioning (HVAC) systems. More specifically, thepresent invention relates to dynamic, digitally implemented HVACcontrol.

[0004] 2. Related Art

[0005] Efforts to manage the environmental condition of a room,building, or other controlled space have resulted in a wide variety ofsystems for controlling the operation of heaters, air-conditioningcompressors, fans, and other components of HVAC equipment. The simplestand most well known form of such control is simply a thermostat whichsenses the temperature of a controlled space, and sends signals to theHVAC system if the temperature is above or below a particular setpoint.Upon receipt of these signals, the HVAC system supplies cooled or heatedair to the space as called for by the thermostat.

[0006] Although this simple system is adequate in many instances,improvements have been and are desired. Many aspects of the developmentof HVAC control apparatus and algorithms focus on increasing occupantcomfort by controlling the environmental condition more tightly. Acompeting concern, however, is minimizing the energy consumed by theHVAC system. It can be appreciated that the various control schemesutilized impact the energy consumption of the HVAC system. In the past,efforts to address excessive energy consumption have focused ondetermining when a space is unoccupied or otherwise has a lowerrequirement for environmental control. Examples of these systems includethose described in U.S. Pat. Nos. 4,215,408 to Games, et al., and5,395,042 to Riley, et al. In U.S. Pat. No. 4,557,317 to Harmon, Jr., anHVAC controller includes a drifting “dead-band”, so that energyconsumption is reduced due to the allowance of wider swings in thetemperature of the controlled space. In the Harmon, Jr. system, occupantcomfort is said to be maintained because the rate of change of thetemperature of the controlled space remains low.

[0007] One potential source of energy savings has thus far not beenfully exploited. This is the minimization of energy loss via heatconduction and radiation through exposed ducting and other components ofthe HVAC system. This energy loss is exacerbated by the fact that acorrectly sized HVAC unit will operate at full capacity only on thehottest or coldest days of the year. The majority of the time, the unitis heating or cooling the supply air to an average temperature which ishotter or colder than that required to meet the demand for environmentalcontrol and maintain comfort for the occupants of the controlled space.This overcapacity results in increased heat transfer from the systemthrough ducting and other mechanical components of the HVAC system.Attempts to recover this escaping energy have thus far been limited. Onesystem attempts to recover escaping energy by extending the operatingperiod of the supply air fan beyond that of the furnace or airconditioner. Another system establishes a fixed duty cycle for thefurnace or air conditioner by measuring the temperature of the air beingsupplied to the controlled space.

[0008] Although these systems do decrease energy waste somewhat,operator comfort is sacrificed to a degree which can be unacceptable.For one thing, existing systems are not responsive to changes inexternal conditions which cause changes in the energy needs of thecontrolled space. Thus, a fixed duty cycle will not be appropriate foroptimally satisfying all calls for heating or cooling. In these cases,the controlled space may require an unacceptably long time to heat orcool to a given thermostat setpoint, leaving the occupants uncomfortablefor an extended period. Furthermore, HVAC cycling during periods of highdemand for heating or cooling may cause noticeable fluctuations in thetemperature of the controlled space.

[0009] In addition to these factors, existing systems do not adequatelyprovide for humidity control. It is recognized that humidity is a factorin occupant comfort as well as temperature. Accordingly, systems whichalter HVAC system operation in response to humidity measurements havebeen produced. One example of such a system, adapted for controlling theair space inside an automobile, is described in U.S. Pat. No. 4,852,363to Kampf, et al. This system includes humidifiers and dehumidifierswhich are operated in response to a humidity measurement. Another morecomplex system, also adapted for control of an automotive HVAC system,is described in U.S. Pat. No. 5,579,994 to Davis, Jr. et al. In theDavis, Jr. device, several environmental parameters are sensed, and anoverall environmental control strategy is developed which is under fuzzylogic control.

[0010] Humidity control may also be performed by cycling an airconditioning unit, as the coils of the air conditioner remove water fromthe air in addition to cooling it. As described in U.S. Pat. No.5,346,129 to Shah et al., an air conditioning system can be run inresponse to relative humidity measurements as well as temperaturemeasurements made in the controlled space. Of course, this may cool theair more than is desired by the occupants of the space, and accordingly,some systems will re-heat the dryer cooled air after it passes thecondenser coils.

[0011] No presently available system, however, reduces HVAC energyconsumption without serious consequences to operator comfort resultingfrom temperature swings and higher humidity levels.

SUMMARY OF THE INVENTION

[0012] An HVAC control apparatus includes input circuitry configured toreceive input signals from external sensors, processing circuitrycoupled to the input circuitry and configured to evaluate the signals,and output circuitry coupled to the processing circuitry. In thisembodiment, the processing circuitry generates output signals whichalter the state of the output circuitry. Input/output circuitrycomprising a wireless transceiver is also coupled to the processingcircuitry for transmitting data to system administrators for diagnosticevaluation of said HVAC system.

[0013] Methods of system management are also provided. In oneenvironmental control system embodiment comprising (1) energy consumingheating and/or cooling components, (2) a digital heating and/or coolingcomponent controller, and (3) system administration facilities, a methodof system diagnostics comprises transmitting information relating to theoperation of the heating and/or cooling components from the digitalcontroller to system administration facilities via a wirelesscommunication link.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of an environmental control systemaccording to one embodiment of the present invention.

[0015]FIG. 2 is a flow chart illustrating the operation of oneembodiment of the present invention.

[0016]FIG. 3 is a schematic/block diagram of an embodiment of anenvironmental control system incorporating aspects of the presentinvention.

[0017]FIG. 4 is a schematic/block diagram of one embodiment of an HVACcontroller according to some aspects of the present invention.

[0018]FIG. 5 is a composite of FIGS. 5A, 5B, SC, and 5D, and is aflowchart illustrating the operation of one embodiment of the presentinvention during a call for heating from a controlled space.

[0019]FIG. 6 is a composite of FIGS. 6A, 6B, 6C, and 6D, and is aflowchart illustrating the operation of one embodiment of the presentinvention during a call for cooling from a controlled space.

[0020]FIG. 7 is a composite of FIGS. 7A, 7B, and 7C, and is a flowchartillustrating another embodiment of the present invention during a callfor cooling from a controlled space.

[0021]FIG. 8 is a composite of FIGS. 8A, 8B, and 8C, and is a flowchartillustrating another embodiment of the present invention during a callfor heating from a controlled space.

[0022]FIG. 9 is a graph of gas use as a function of time for an HVACsystem which is operated in accordance with principles of the presentinvention.

[0023]FIG. 10 is a graph illustrating instantaneous and average energyoutput to a controlled space when the HVAC system is operated in heatingmode in accordance with principles of the present invention.

[0024]FIG. 11 is a graph of input power as a function of time for anHVAC system which is operated in accordance with principles of thepresent invention.

[0025]FIG. 12 is a graph illustrating instantaneous and average energyoutput to a controlled space when the HVAC system is operated in coolingmode in accordance with principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Preferred embodiments of the present invention will now bedescribed with reference to the accompanying Figures, wherein likenumerals refer to like elements throughout. The terminology used in thedescription presented herein is intended to be interpreted in itsbroadest reasonable manner, even though it is being utilized inconjunction with a detailed description of certain specific preferredembodiments of the present invention. This is further emphasized belowwith respect to some particular terms used herein. Any terminologyintended to be interpreted by the reader in any restricted manner willbe overtly and specifically defined as such in this specification.

[0027] Referring now to FIG. 1, an environmental control systemaccording to some aspects of the present invention is illustrated. Acontrolled space 10 receives heated and/or cooled air from a heating,ventilation and air conditioning (HVAC) unit 20. The controlled spacemay be an automobile interior, an office building, a barn or otheranimal enclosure, a computer room, or any other space for whichenvironmental control is advantageous. Supply air may flow to thecontrolled space 10 via an air supply duct 12. Return air from thecontrolled space 10 is routed back to the HVAC unit 20 via an air returnduct 14. The HVAC unit 20 typically comprises an oil or gas furnace forheating the air of the controlled space 10 as well as an airconditioning unit for cooling the air of the controlled space 10. TheHVAC unit 20 may also comprise vents and ducting (not shown) for drawingoutside air into the system. The HVAC system shown and described hereinincludes both air heating and air cooling apparatus, and is typical ofmany common installations. It will be appreciated that the term “HVAC”as used herein also includes stand alone heaters, stand alone airconditioners, heat pumps, and other equipment that perform some or allof the environmental control functions for a controlled space.

[0028] Coupled to the HVAC unit 20 is an HVAC controller 30. The HVACcontroller 30 may receive information regarding environmental conditionsand other information from the controlled space 10 via a sensor signalpath 22. The sensor signals may comprise electrical signals fromthermocouples, thermistors, electronic humidity sensors, and othersensor types well known to those in the art. The sensor signals may alsocomprise signals from the controlled space 10 calling for heating,cooling, or providing other information about the condition and needs ofthe controlled space 10. Another signal path 24 is provided between theHVAC controller 30 and the HVAC unit 20. This signal path preferablyincludes control signals from the HVAC controller 30, and may furtherinclude sensor signals which are routed back to the HVAC controller 20.It can be appreciated that both signal paths are not always necessary todeliver the required information to the HVAC controller 30.

[0029] The HVAC controller 30 preferably operates to dynamically controlthe on/off state of components of the HVAC unit 20 to recover what wouldbe wasted energy during those times when the HVAC unit 20 can meet thedemands of the controlled space without operating at maximum capacity.In some embodiments, information concerning the condition of thecontrolled space and information concerning the condition of the air theHVAC unit 20 is supplying to the controlled space 10 is received by theHVAC controller 30 via one or both signal paths 22 and 24. Thisinformation is used to determine whether or not the HVAC unit 20 canmeet the heating or cooling demands of the controlled space 10 at a lowenergy consumption rate while maintaining occupant comfort.

[0030]FIG. 2 is a flowchart illustrating one possible mode of operationfor an HVAC system as shown in FIG. 1. In the first step 32, the HVACcontroller waits for a call for environmental modification from thecontrolled space 10. This signal may be sent by a thermostat inside thecontrolled space 10 to the HVAC controller 30 along signal path 22 ofFIG. 1. When a call is sensed, heat transfer begins to or from thecontrolled space at step 34. For example, when a call for heating isreceived, a gas burning furnace may be started, and heat energy will betransferred to the controlled space by ventilating the controlled spacewith heated air. When a call for cooling is received, heat energy istransferred from the controlled space to the outside environment by acooling coil system as is well known in air conditioning systems.

[0031] At step 36, the HVAC controller receives and evaluatesinformation regarding conditions in the HVAC system. As will beexplained further below, these conditions may advantageously includevarious environmental and physical conditions and parameters such as theapproximate temperature of the controlled space and the rate of changeof that temperature, the approximate temperature of the air beingsupplied to the controlled space and the rate of change of thattemperature, the approximate humidity of the controlled space, thelength of time the HVAC system has been in an on or off state, thelength of time a call for heating or cooling has been pending, etc.Based on the present disclosure, those of skill in the art willappreciate that not all of these parameters need to be evaluated to makeor use the present invention. In addition, other parameters notspecifically mentioned may be used when, for example, certain parametersare especially relevant to a particular installation. The terms“physical” or “environmental” parameters or conditions are thus intendedto include a wide variety of information concerning the operation andstatus of the HVAC system and related devices and locations, and notsimply the several described in detail herein with respect to certainspecific embodiments of the present invention. As mentioned above, someor all of this information may be transferred from sensors to the HVACcontroller 30 along signal paths 22 and/or 24 of FIG. 1.

[0032] The next step 38 involves the determination of whether or not therate of energy transfer to or from the controlled space can be reducedwhile meeting the call for environmental change within certain specifiedrequirements. In preferred embodiments, at least some of therequirements are designed to ensure that occupant comfort is notsacrificed to an unacceptable degree when reducing the rate of energytransfer. As will be explained below, however, occupant comfort is notthe only consideration at step 38. HVAC system operation requirementssuch as the prevention of over-cycling an air conditioning compressormay also be considered at step 38. In some advantageous embodiments ofthe present invention, the decision of step 38 is made based on theevaluation of physical and/or environmental parameters and conditionsperformed at step 36.

[0033] At step 40, if the energy transfer rate can be reduced within thespecified requirements, the system will initiate a reduction in theaverage heat energy transfer rate. In some embodiments, this step willinvolve shutting off some energy consuming portion of the HVAC system.For example, if the HVAC system is currently cooling the controlledspace, the system may turn off the air conditioning compressor and/oroutdoor fan. Furthermore, if the HVAC system is currently heating thecontrolled space, the system may close a valve which supplies gas to afurnace. Most preferably, the HVAC system continues to ventilate thecontrolled space, even though one or more energy consuming componentshave been turned off. This continued ventilation is advantageous becausewhile the energy consuming component such as the compressor or outdoorfan is off, the continued ventilation allows heat transfer to continueby recovering energy from system components such as the ducting, othermechanical components of the HVAC system, and structural elements of thecontrolled space. Thus, energy consumption may be reduced, but usefulheat transfer may continue for a certain period of time.

[0034] If, on the other hand, the system determines that the energytransfer rate cannot be reduced consistent with certain environmentaland/or operational requirements, at step 42 the system will transferheat energy at the maximum rate. In some embodiments, the decision totransfer heat energy at the maximum rate will be based on considerationssuch as a very low temperature in the controlled space when heating iscalled for, a very high temperature in the controlled space when coolingis being called for, or insufficient changes over time in the controlledspace temperature when the controlled space is calling for heating orcooling.

[0035] As illustrated by step 44 in FIG. 2, system operation alsodepends on whether or not the call for environmental modification isstill pending. If the call is no longer pending, at step 46 the HVACsystem will shut down, and the system waits for the next call forenvironmental modification back up at step 32. If the call is stillpending, the physical and environmental conditions of the system areagain evaluated at step 36, and a decision at step 38 is made regardingwhether or not the energy transfer rate can be reduced or should be setto the maximum rate.

[0036] It can be appreciated that at any given instance of executingstep 38, the HVAC system may be in a state of reduced or maximum energytransfer depending on the results of any prior evaluations which havebeen performed, how long the system has been operating, and otherfactors. Whichever state the system is currently in, however, it isadvantageous to conduct frequent reevaluations of the physical andenvironmental parameters to determine whether or not the currentoperation mode is optimal. Thus, dynamic control of energy consumptionis produced. This in turn allows for increased energy efficiency whilemaintaining an acceptable level of comfort for occupants of thecontrolled space.

[0037] It will also be appreciated by those of skill in the art that inmany embodiments, the steps illustrated in FIG. 2 may be performed invarious orders other than that explicitly shown. In addition, the steps36 and 38 will, in some embodiments, be implemented as an essentiallycontinuous process of comparing sensor inputs to preset limits, andchanging the operating mode of the system by performing either step 40or 42 as indicated when a sensor input signal reaches one of the limits.It may also be noted that many different ways of implementing the twooperating modes of steps 40 and 42 may be implemented. For example, step40 may define an “off” mode for some HVAC components, while step 42defines an “on” mode for those components. In this case, the duration ofeach on or off period may vary depending on the values of the physicaland environmental parameters sensed at step 36. Alternatively, step 40could define a mode of operation where the HVAC system enters an on/offcycled mode at a particular duty cycle. In this case, the duty cycle mayvary depending on the values of the physical and environmentalparameters sensed at step 36. In both of these embodiments, the mode ofoperation entered into following step 40 is an energy consumptionreduction mode. In some embodiments, this mode also comprises an energyrecovery mode which increases the overall efficiency of operation of theHVAC system.

[0038]FIG. 3 illustrates one specific embodiment of an apparatusconstructed according to some aspects of the present invention. In FIG.3, an HVAC system 48 is shown which comprises several components. Asupply air fan 50 (sometimes called an “indoor fan” even though it mayactually be mounted outdoors) ventilates a controlled space by forcingair through an air supply duct 52 into a controlled space. The supplyair fan 50 also forces ambient air from the controlled space back intothe HVAC unit 48 via a return duct 54. In a chamber 56 in the HVACsystem 48, this return ambient air may be heated or cooled before it isreturned to the controlled space via the air supply duct 52. Toaccomplish this, the chamber 56 includes air conditioner cooling coils58 and a gas fired furnace 60. The furnace 60 includes an electricallyactuable valve 64 in its gas supply line 62. The cooling coils 58 arecoupled to a compressor 66 and heat exchanger coils 68 which areadjacent to a fan 70. These components of the HVAC system may beconventional, and their construction and operation will not be furtherdescribed herein. Furthermore, it will be appreciated that HVAC systemscome in a variety of forms, all of which may be used with the presentinvention. For example, a given installation may use a split system withseparate furnaces and air conditioners. Systems with several heating orcooling stages may also be used. Heat pumps are another form of energycontrol system that is compatible with the present invention.

[0039] The AC power input lines 72 are routed to the components of theHVAC unit 48 through three relays 74, 76, and 78. The relay contacts ofrelay 74 are connected between the AC power and the fan 70. The relaycontacts of relay 76 are connected between the AC power and thecompressor 66. The relay contacts of relay 78 are connected between theAC power and the air supply fan 50. One side of the coils of relays 74,76, and 78 are tied to ground via their common connection to a groundedline 80. The other side of relay coils 74 and 76 are connected to a line82 which is routed through a terminal block 84 and to an HVAC controller86, which is illustrated in more detail in FIG. 4. The electricallyactuated gas valve 64 also has one line connected to the ground line 80,and another line 88 which is also connected to the HVAC controller 86through the terminal block 84. Thus, by placing and removing anappropriate voltage on lines 82 and/or 88, the HVAC controller may turnthe compressor 66, fan 70, and the gas valve 64 on and off.

[0040] Although a power source for this voltage may be created in manydifferent ways known in the art, one convenient method illustrated inFIG. 3 is to use a 24 V output step down transformer 90, which has itsinput connected to the AC input lines, and its 24 V output connected tothe HVAC controller 86 and other system components which require 24 Vpower.

[0041] In addition, the coil of the third relay 78 has one sideconnected to the ground line 80. The other side of the coil of the relay78 is connected to line 92, which is routed through the terminal block84 and to a thermostat 94. The thermostat 94 is typically mounted in thecontrolled space. Thus, a thermostat output signal may directly controlthe operation of the supply air fan 50, without modification orinterruption by the controller 86. In typical systems, the thermostatwill activate the indoor fan 50 continuously for the entire duration ofany call for heating or cooling.

[0042] As will also be described in more detail with reference to FIG.4, the HVAC controller receives signals from the thermostat 94 or otherremote device on lines 96 and 98. Through these signal lines 96, 98, theHVAC controller receives calls for heating and cooling from thecontrolled space. Other information may also be transferred through, asone example, additional lines 97, 99. These two lines may be provided toindicate whether or not second stage heating or cooling has beenactivated by the system. Additional signals representative of othersystem parameters may also be provided. The response of the system tothese signals is described in relation to specific embodiments of thepresent invention in more detail below with reference to FIGS. 5 through8.

[0043] The HVAC controller also receives signal inputs from sensors. Onesensor 100 may be located in the air supply duct 52. In someembodiments, this sensor 100 has an output signal representative of theapproximate temperature of the air being supplied to the controlledspace by the HVAC system. This output signal is routed to the HVACcontroller via line 104. The output signal may advantageously comprise atwo-wire thermistor or thermocouple signal.

[0044] A second sensor 102 may be located in the return duct 54. Thissensor 102 preferably has an output signal representative of theapproximate temperature of the ambient air returning from the controlledspace. Also, the sensor 102 may sense the approximate humidity of theambient air returning from the controlled space, and have a secondoutput signal representative of this parameter. This output signal,which may advantageously comprise a four wire interface to the HVACcontroller, two for the thermistor or thermocouple and two for thehumidity sensor, is routed to the HVAC controller on line 106. Inaddition, the sensor 102 may comprise a carbon monoxide sensor. In thiscase, the HVAC controller can be made to signal an audible alarm and/orshut off gas flow to the furnace 60 if excessive carbon monoxide levelsare sensed.

[0045] It will be appreciated that the sensors 100 and 102 may belocated in locations different from that shown and still perform thefunction required. As a specific example, the sensor 102 could belocated in the controlled space itself to measure the ambienttemperature and humidity. The temperature in the return air duct issimply a convenient substitute or proxy for this usually more remotelocation.

[0046]FIG. 4 provides a more detailed illustration of the HVACcontroller 86 shown in FIG. 3. In this embodiment, the HVAC controllercomprises a microprocessor 120. The term “microprocessor” in thisapplication is intended to include any of a variety of digital processorconfigurations, including the commercially available microprocessorssuch as the X86 family from Intel. In many preferable embodiments, themicroprocessor is a commercially available microcontroller or digitalsignal processor available, for example, from Motorola or TexasInstruments.

[0047] The microprocessor 120 is coupled to the sensor inputs 104 and106 through analog signal conditioning and optical isolation circuitry122 and an analog to digital converter 124 to provide digitized datarepresentative of the environmental conditions sensed by the sensors100, 102. The microprocessor 120 is also coupled to inputs 96, 97, 98,99 from a thermostat in the controlled space or another remote device.In some advantageous embodiments, these signals comprise two levelinputs, i.e. ground and a nominal voltage, typically 24 VAC, or perhaps5 Vdc for a digital system. For example, a call for cooling will beindicated by line 96 being asserted by being pulled to the nominalvoltage. A call for heating will be indicated by line 98 being assertedby being pulled to the nominal voltage. Analogously, assertion of line97 may indicate that secondary cooling has been activated, and assertionof line 99 may indicate that secondary heating has been activated. Thesesignals are coupled to the microprocessor 120 through signalconditioning circuitry 126. In some embodiments, these signals may beoperable to interrupt microprocessor operation. In this case, wheneverlines 96 or 98, for example, are unasserted, the processor senses thatno call for heating or cooling is being made, and therefore halts anyongoing control operation and waits for the next call to re-initiatecontrol over the HVAC system components.

[0048] The microprocessor 120 is also coupled to a memory 128. Thismemory may store previously received digital data obtained from theinputs 96, 97, 98, 99, 104, and 106, the time at which such data wasreceived, the length of time the compressor or furnace has been on oroff, and other information relevant to HVAC operation. Some of thisinformation may be produced at least in part by a timer 121 implementedwithin the microprocessor or as a discrete clock device. In manyadvantageous embodiments, the timer 121 will not generate an absolutereal time, but will be configured to measure time spans relative to someprior event such as the initiation of a call for heating or cooling.Also stored in memory 128 are predetermined setpoints against which suchdata is compared to make decisions regarding HVAC operation. It can betherefore appreciated that the memory 128 advantageously may include anon-volatile portion such as EEPROM memory as well as RAM memory. EEPROMmay be advantageous in that no backup battery is required.

[0049] The microprocessor further interfaces with an I/O port 130 forcommunicating information about the environmental and physicalparameters being monitored, and the status of the HVAC system. Thisinformation is valuable to system administrators in evaluating systemperformance and in troubleshooting system malfunctions. In addition, themicroprocessor can be re-programmed by altering stored setpoints via theI/O port 130. The I/O port 130 may advantageously comprise an RS232serial port well known to those in the art to make communication withwidely available personal computers and handheld palmtop computersconvenient. If desired, the I/O port may comprise a wirelesstransceiver, and/or may interface to a modem for system monitoring andcontrol via RF and/or telephone communication links.

[0050] The microprocessor 120 may additionally include two outputs 132,134 which, after some buffering, drive the coils of two normally closedoutput relays 136, and 138 respectively. The contact of one of theserelays 136 is configured to output the thermostat heating call line 98to the output line 88 (see FIG. 3) which controls the gas valve 64. Thecontact of the other relay 138 is configured to output the thermostatcooling call line to the output line 82 (see FIG. 3) which controls thecompressor 66 and fan 70. Thus, the operation of the gas valve 64, thecompressor 66, and the fan 70 may be controlled by the microprocessor120 by selectively opening the contacts of the relays 136, 138. In theembodiment of FIG. 4, the normally closed relays 136 and 138 ensurenormal operation of the HVAC system if the controller is powered down oris otherwise non-operational. In this case, the heating and coolingcalls pass through the relays 136, 138, and actuate the compressor, fan,and furnace as in a conventional HVAC system.

[0051] Although more detail is provided below with regard to specificimplementations of controller operation, certain fundamental propertiescan be appreciated from examination of FIGS. 3 and 4. For instance, themicroprocessor 120 may be configured to take digital data representativeof environmental and physical conditions, make operational decisionsbased on those conditions, and dynamically control the on/off state ofenergy consuming components such as the compressor 66 and furnace 60based on the operational decisions made. The digitally based decisionmaking allows for a wide variety of sensor inputs on which to baseoperational decisions. The HVAC controller can also be convenientlyprogrammed via the I/O port for easy customization to different systemsand alterations to existing installations.

[0052] It can be appreciated that many alternative methods of systemcontrol can be used to improve HVAC efficiency by monitoring physicaland environmental parameters of the system and the associated controlledspace. In FIGS. 5 through 8, specific implementations of dynamicallycontrolled energy recovery during heating and cooling calls areillustrated. The implementations shown may be advantageously producedwith appropriate configuration, via programming, of the microprocessorof FIG. 4. In the discussion below with reference to these Figures,several specific time periods, setpoints, and other parameters aredescribed. Although the specific parameters mentioned have been foundsuitable, it will be appreciated that a wide variety of options forthese parameters are possible within the scope of the present invention.Furthermore, for clarity of explanation, some of the steps set forthbelow are described in terms of operations of the apparatus of FIGS. 3and 4. This apparatus is advantageous in implementing the describedcontrol procedures, but it will be appreciated that many different typesof physical hardware may be used to perform the functions described.

[0053] One implementation of actions during a call for heating areillustrated in FIG. 5, beginning at step 150 of FIG. 5A, where thecontroller determines whether a heating call is being made. If not, thesystem waits for a call at step 152. Once a call for heating has beenmade, the controller then initiates a heating call timer at step 154 tokeep track of how long this particular call for heating has beenpending. This measurement may be used later in the HVAC control process.Also, at step 156, the system sets a minimum run timer to four minutesand starts the minimum run timer at step 158. Heating is initiated atstep 160 by asserting line 88 of FIGS. 3 and 4 to open the gas valve 64.It will be appreciated that step 160 is performed immediately uponreceipt of the call for heating when apparatus in accordance with FIG. 4is utilized as an HVAC controller. This is because the normally closedrelay 138 sends the call to the gas valve when it is received.

[0054] Once heated air begins flowing to the controlled space, thecontroller monitors the approximate air supply temperature T_(s), andcompares it to a predetermined maximum setpoint, which will typically bein the range of 120 to 140 degrees Fahrenheit. As it generally takessome time for the supply air temperature to reach this value, thiscomparison initially results, at step 162, in a decision that the supplyair temperature is less than the setpoint. In this case, at step 164,the controller then compares the rate of change of T_(s) with its mostrecent past value. If the rate of change of T_(s) has increased, thecontroller takes no action, waiting for 15 seconds at step 166, andloops back up to step 162 to again compare the supply air temperaturewith the predetermined maximum setpoint. If the rate of change of T_(s)is not increasing, at step 168 of FIG. 5B, the controller compares therate of change of T_(s) with another predetermined setpoint, which maybe set at approximately 0.5 to 5 degrees Fahrenheit per minute. If therate of change of T_(s) is more than this setpoint, the controller againperforms no action and waits 15 seconds at step 166. If T_(s) is greaterthan its setpoint, or the rate of change of T_(s) is less than itssetpoint, at step 170 the controller checks if the minimum run timerstarted at step 158 has timed out. If not, the controller again waits 15seconds at step 166. Thus, after initiating heating, the controllerperforms no action until the minimum run timer has timed out, and eitherT_(s) is above its setpoint, or the rate of change of T_(s) is below itssetpoint. The minimum run timer thus ensures that the heating continuesin an on state for at least an amount of time which is consistent withthe manufacturers' specifications.

[0055] Once these conditions are met, the controller determines at step172 whether or not it is receiving a signal indicating that secondaryheating is also being utilized in a two stage HVAC system. Thisinformation may be received on line 99 of FIG. 4 for example. Ifsecondary heating has been activated, it indicates that no reduction inenergy transfer for the first stage coupled to the controller shouldtake place. The controller will therefore, if second stage heating isrequired to satisfy the call, loop back to continue monitoring T_(s) andits rate of change.

[0056] At step 174, the controller checks to see if the call is stillpending. If not, the heat transferred has satisfied the call, andheating should be discontinued. In control systems implemented withapparatus constructed as shown in FIG. 4, it can be seen that as soon asthe call from the thermostat is satisfied, operation of the furnace willstop, because the call signal on line 98 will no longer be present to berouted to the gas valve through the associated relay 138. As alsodescribed above with respect to the apparatus of FIG. 4, the step ofchecking for pending calls may be implemented by interrupting processoroperation when deassertion of, for example, line 98 is sensed by themicroprocessor.

[0057] If, however, at step 174 it is determined that the call is stillpending, the controller evaluates the amount of time the call has beenpending. Referring now to FIG. 5C, if at step 182 it is determined thatthe call has been pending for more than 15 minutes, at step 184 theminimum run timer is reset to eight minutes. If, at step 186, it isdetermined that the call has been pending for more than 30 minutes, atstep 188 the minimum run timer is reset to twelve minutes. If, at step190, it is determined that the call has been pending for more than 60minutes, at step 192 the controller will loop back to step 162 tocontinue monitoring T_(s) and its rate of change, thereby avoidingentering a mode of reduced energy consumption.

[0058] As mentioned above, the minimum run timer is initially set tofour minutes, so in the beginning, the pending call time will likely notsatisfy the 15, 30, and 60 minute tests defined in steps 182, 186, and190, unless other requirements such as are imposed on the supply airtemperature were not met in a short time after cooling began. Thecontroller will therefore likely not initially reset the minimum runtimer, and at step 192, checks the ambient air temperature of the spaceby looking at T_(r), the temperature of the air in the return duct. Ifthis temperature is lower than 55 degrees F., the controller again loopsback to step 162 to continue monitoring T_(s) and its rate of change.However, if T_(r) is greater than 55 degrees F., the controller willinitiate energy recovery mode at step 194. This step of comparing T_(r)with a fixed value allows the system to inhibit energy recovery when thethermostat setpoint is far different from the actual temperature of thecontrolled space. The HVAC unit will thus operate at maximum outputuntil the temperature of the controlled space reaches a more comfortablevalue.

[0059] Referring back to FIGS. 3 and 4, in this embodiment the enteringof energy recovery mode may involve simply the opening of the valve 64by opening the relay 98 to remove the call signal from line 88. Thisreduces the energy consumption of the HVAC unit dramatically. However,the supply air fan remains operational, so that air can continue to flowthrough the system, drawing heat from system components that wouldotherwise be radiated or conducted away and lost. As shown by FIG. 5,this energy recovery step 194 is taken if (1) the minimum run timer issatisfied, (2) either T_(s) is greater than its maximum setpoint or therate of change of T_(s) is less than its minimum setpoint, and (3) thetemperature of the controlled space is greater than 55 degrees F.Otherwise, the heating initiated at step 160 is continued.

[0060] Following the initiation of recovery at step 194, the short cycletimer is started at step 196. Once recovery is initiated at step 194 andthe furnace is off, the air in the supply duct begins to cool off, andto draw heat from the ducting material, other mechanical components ofthe HVAC system, and structural components of the controlled space. Thisenergy recovery continues as the supply air temperature drops toward theambient temperature, and the controller will wait until certainconditions are met before re-initiating heating.

[0061] Therefore, at step 198 of FIG. 5D, the controller then determineswhether or not the supply air temperature is above a minimum setpoint,typically set at 90 to 100 degrees F. If it is, the controller thenchecks, at step 200, if the rate of change of the temperature of thecontrolled space is positive, that is, is the controlled space stillgetting warmer. If it is, the controller moves to step 202, anddetermines if the rate of change (in the negative direction this time)of the supply air temperature has increased over that previouslyrecorded. If it has, the controller takes no action, and waits 15seconds at step 204 before looping back to step 198 and re-checking thesupply air temperature.

[0062] If the rate of change is not decreasing, the controller thenchecks at step 206 if the rate of change of the supply air temperatureis greater than a predetermined setpoint, which may advantageously beset at 0.5 to 5 degrees F. per minute. If it is, the controller againtakes no action, and waits 15 seconds at step 204 before looping back tostep 198 and re-checking the supply air temperature.

[0063] If any one of these three conditions hold: (1) at step 198 T_(s)is less than the minimum setpoint, (2) at step 200 the controlled spaceis no longer increasing in temperature, or (3) the rate of change ofT_(s) is less than a predetermined setpoint, then the controller willmove out of this 15 second increment waiting loop and first check atstep 208 to see if the call for heating is still pending. If the heatingcall has been satisfied (i.e., the call is no longer pending), thesystem loops up to state 150, and waits for the next call. If theheating call has not been satisfied, heating should be re-initiated. Inthis case, the short cycle timer, which was started at step 196, ischecked at step 210 to see if it is timed out. If not, the controllerthen waits, in five second increments illustrated by step 212, for theshort cycle timer to time out. When it has been determined that theshort cycle timer timed out at step 210, the controller loops back tostep 160 and reinitiates a heating cycle by turning the gas valve forthe furnace back on, by, for example, allowing the relay 138 to close,and outputting the call signal on line 88 again.

[0064]FIG. 6 illustrates a specific implementation of energy recoveryduring a cooling call according to aspects of the present invention.Once again, the implementation shown is advantageously produced withappropriate configuration, via programming, of the microprocessor ofFIG. 4. Many parallels to the heating flowchart of FIG. 5 will beapparent, although some differences exist.

[0065] Actions during a call for cooling are illustrated in FIG. 6,beginning at step 220 of FIG. 6A, where the controller determineswhether a cooling call is being made. If not, the system waits at step222. Once a call for cooling has been made, the controller theninitiates a cooling call timer at step 224 to keep track of how longthis particular call for cooling has been pending. This measurement maybe used later in the HVAC control process. Also, at step 226, the systemsets a minimum run timer to four minutes and starts the minimum runtimer at step 228. Cooling is initiated at step 230 by asserting line 82of FIGS. 3 and 4 to turn on the compressor 66 and fan 70. It will beappreciated that step 230 is performed immediately upon receipt of thecall for cooling when apparatus in accordance with FIG. 4 is utilized asan HVAC controller. This is because the normally closed relay 136 sendsthe call to the compressor and outdoor fan when it is received.

[0066] Once cooled air begins flowing to the controlled space, thecontroller monitors the approximate air supply temperature T_(s), andcompares it to a predetermined minimum setpoint, which will typically bein the 50 to 60 degrees Fahrenheit range. As it generally takes sometime for the supply air temperature to reach this value, this comparisoninitially results, at step 232, in a decision that the supply airtemperature is greater than the setpoint. In this case, at step 234, thecontroller then compares the rate of change of T_(s) with its mostrecent past value. If the rate of change of T_(s) increased, thecontroller takes no action, waiting for 15 seconds at step 236, andloops back up to step 232 to compare the supply air temperature with thepredetermined minimum setpoint. If the rate of change of T_(s) is notincreasing, at step 238 of FIG. 6B the controller compares the rate ofchange of T_(s) with another predetermined setpoint, typically set at0.5 to 5 degrees Fahrenheit per minute. If the rate of change of T_(s)is more than this setpoint, the controller again performs no action andwaits 15 seconds at step 236.

[0067] If either T_(s) is greater than its setpoint, or the rate ofchange of T_(s) is less than its setpoint, at step 240 the approximatehumidity of the controlled space may be checked. If this humidity isgreater than a maximum setpoint, the controller again waits 15 secondand loops back to step 232. Cooling will thus continue at maximum outputduring high humidity periods. If the humidity is lower than thesetpoint, the controller determines at step 242 whether or not it isreceiving a signal indicating that secondary cooling is also beingutilized in a two stage HVAC system. This information may be received online 97 of FIG. 4 for example. If secondary cooling has been activated,it indicates that no reduction in energy transfer for the first stagecoupled to the controller should take place. The controller willtherefore, if second stage cooling is required to satisfy the call, loopback to continue monitoring T_(s) and its rate of change.

[0068] If second stage cooling is not activated, the status of the callfor cooling is checked at step 244. If the heat transferred hassatisfied the call, cooling should be discontinued. In control systemsimplemented with apparatus constructed as shown in FIG. 4, it can beseen that as soon as the call from the thermostat is satisfied,operation of the compressor and fan will stop, because the call signalon line 96 will no longer be present to be routed to the compressor 66and fan 70 through the associated relay 136. The system thus loops backto step 220 and awaits the next call for cooling. As also describedabove with respect to the apparatus of FIG. 4, the step of checking forpending calls may be implemented by interrupting processor operationwhen deassertion of, for example, line 96 is sensed by themicroprocessor.

[0069] Moving back to step 244, if the cooling call has not beensatisfied, the controller waits for the minimum run timer to time out bychecking its status at step 256, and waiting in five second incrementsat step 258 until the minimum run timer has timed out. The minimum runtimer ensures that the compressor operates for a time at least as longas suggested by the compressor manufacturer before entering a recoverycycle.

[0070] Once the minimum run timer has expired, the controller moves tostep 260 of FIG. 6C and evaluates the amount of time the call has beenpending. If, at step 260, it is determined that the call has beenpending for more than 15 minutes, at step 262 the minimum run timer isreset to eight minutes. If, at step 264, it is determined that the callhas been pending for more than 30 minutes, at step 266 the minimum runtimer is reset to twelve minutes. If, at step 268, it is determined thatthe call has been pending for more than 60 minutes, the controller willloop back to step 232 to continue monitoring T_(s) and its rate ofchange, bypassing entry into an energy recovery mode.

[0071] As mentioned above, the minimum run timer is initially set tofour minutes, so in the beginning, the pending call time will likely notsatisfy the 15, 30, and 60 minute tests defined in steps 260, 264, and268, unless other requirements such as are imposed on the supply airtemperature were not met in a short time after cooling began. Thecontroller will therefore likely not initially reset the minimum runtimer, and at step 270, checks the ambient air temperature of thecontrolled space by looking at T_(r), the temperature of the air in thereturn duct. If this temperature is higher than 80 degrees F., thecontroller again loops back to step 232 to continue monitoring T_(s) andits rate of change. However, if T_(r) is less than 80 degrees F., thecontroller will initiate energy recovery mode at step 272. In this caseas well, recovery mode is not entered if the temperature of thecontrolled space is uncomfortable.

[0072] Referring back to FIGS. 3 and 4, in this embodiment energyrecover mode may involve simply the shutting down of the compressor 66and fan 70 by removing the cooling call signal from line 82 by openingthe associated relay 136. This reduces the energy consumption of theHVAC unit dramatically. However, the supply air fan remains operational,so that air can continue to flow through the system, losing heat tosystem components that would otherwise remain in the controlled spacewhich is being cooled. As shown by FIG. 5, this step 272 is taken if (1)the minimum run timer is satisfied, (2) either T_(s) is less than itsminimum setpoint or the rate of change of T_(s) is less than its minimumsetpoint, and (3) the temperature of the controlled space is less than80 degrees F. Otherwise cooling initiated at step 230 is continued.

[0073] Following the initiation of recovery at step 272, the short cycletimer is started at step 274. Once recovery is initiated and thecompressor 66 is off, the air in the supply duct begins to warm, and tolose heat to the cold ducting material, other mechanical components ofthe HVAC system, and structural elements of the controlled space. Energyrecovery thus continues as the supply air temperature warms toward theambient temperature, and the controller will wait until certainconditions are met before re-initiating cooling.

[0074] Referring now to FIG. 6D, at step 276 the controller thendetermines whether or not the supply air temperature is below a maximumsetpoint, typically set at 60 to 70 degrees F. If it is, the controllerthen checks, at step 278, if the rate of change of the temperature ofthe controlled space is negative, that is, is the controlled space stillgetting cooler. If it is, the controller moves to step 280, anddetermines if the rate of change (in the positive direction this time)of the supply air temperature has increased over that previouslyrecorded. If it has, the controller takes no action, and waits 15seconds at step 282 before looping back to step 276 and re-checking thesupply air temperature.

[0075] If the rate of change is not decreasing, the controller thenchecks at step 284 if the rate of change of the supply air temperatureis greater than a predetermined setpoint, which may be set at 0.5 to 5degrees per minute. If it is, the controller again takes no action, andwaits 15 seconds at step 282 before looping back to block 276 andre-checking the supply air temperature.

[0076] If any one of these three conditions hold: (1) at step 276 T_(s)is more than the maximum setpoint, (2) at step 278 the controlled spaceis no longer decreasing in temperature, or (3) the rate of change ofT_(s) is less than a predetermined setpoint, then the controller willmove out of this 15 second increment waiting loop and first check atstep 286 to see if the call for cooling is still pending. If the coolingcall has been satisfied (i.e., the call is no longer pending), thecontroller loops back to block 220 to wait for the next call forcooling. If the cooling call has not been satisfied, cooling should bere-initiated. In this case, the short cycle timer, which was started atstep 274, is checked at step 288 to see if it is timed out. If not, thecontroller then waits, in five second increments illustrated by step290, for the short cycle timer to time out. When it has been determinedthat the short cycle timer timed out at step 288, the controller loopsback to step 230 and reinitiates a cooling cycle by turning the airconditioning compressor back on, by, for example, allowing the relay 138to close, and outputting the call signal on line 88 again. The shortcycle timer therefore prevents the restart of the compressor for aperiod at least as long as that recommended by the compressormanufacturer.

[0077] Another alternative control procedure for cooling is illustratedin FIGS. 7A through 7C. As with the procedures of FIGS. 5 and 6, thesystem begins in FIG. 7A at step 300 monitoring whether or not a callfor cooling is being made. If not, the system waits for a call at step302. If a call for cooling is has been received, the system moves toblock 304, and checks the approximate humidity of the controlled spaceor, as explained above, the approximate humidity in the return air duct.If the measured humidity is greater than 60%, the controller isinhibited from initiating energy recovery, and conventional coolingcontrol is performed at step 306. Typically, in the conventional controlmode of block 306, the system simply cools at maximum capacity for theduration of any call for cooling, and is shut off otherwise. As oneexample, if the apparatus of FIGS. 3 and 4 is used to implement thismethod, the processor 120 is inhibited from opening either relay 136,138.

[0078] If the humidity is below 60%, a cooling minimum run timer is setand started. If the call has just been received, and no prior energyrecovery cycles have taken place, at step 308 a minimum run timer forthe first cooling cycle is set and started. As will be discussed furtherbelow, if the system is returning from a recovery cycle, a minimum runtimer of possibly different duration is set and started at step 310.Although suitable systems may be created using a run timer of the sameduration for all cycles, it may be desirable for compressor operation ifthe first cooling cycle, which may follow lengthy off period, issomewhat longer than the cooling periods between recovery cycles. Theminimum run timer of block 308 thus only affects compressor operationduring the first cycle after receiving a call for cooling.

[0079] Moving now to block 312, the controller computes a targettemperature for the air returning from the controlled space. Thistemperature is computed to ensure that the controlled space temperatureis reduced by a minimum amount prior to the initiation of energyrecovery. In some embodiments of the present invention, the targettemperature may be calculated with the current return air temperature, auser programmable minimum cooling rate (which may advantageously be setto 2 to 5 degrees F. per hour) and the initial set value of the minimumrun timer, which may advantageously also be user programmable. In oneembodiment, the target temperature is calculated by calculating thetemperature reduction produced by a cooling of the air produced bymaintaining the user programmed minimum rate for the duration of theminimum run timer initial setting. For example, if the programmableminimum cooling rate is 3 degrees per hour, and the minimum run timer isset to 6 minutes, the target temperature is set to 3 degrees per hourtimes 0.1 hours, or 0.3 degrees cooler than the current return airtemperature.

[0080] At step 314 cooling is initiated. As discussed above withreference to FIGS. 5 and 6, this will occur immediately upon receipt ofthe call if the apparatus implementing this procedure is made inaccordance with FIG. 4.

[0081] While the air conditioner is activated, at block 316 the systemmeasures the temperature of the air in the controlled space. Asmentioned above, this measurement can be made by directly sensingtemperature in the controlled space, or by sensing the temperature in areturn air duct. If the return temperature has not cooled to the targettemperature, the controller performs no further action and at block 318waits five seconds before making another measurement at block 316. Thus,the controller remains in the loop defined by blocks 316 and 318 untilthe approximate temperature of the air in the controlled space dropsbelow the target temperature.

[0082] Referring now to FIG. 7B, once the approximate temperature of thecontrolled space drops below the target temperature, the system checksif the cooling call is satisfied at step 320. If it is, the systemreturns to block 300 on FIG. 7A, and waits for the next cooling call. Ifthe cooling call has not been satisfied, at steps 322 and 324 thecontroller checks the status of the minimum run timer which was set andstarted at step 308 (or step 310 if this is not the first cyclefollowing a call for cooling) described above. As long as this timer hasnot expired, the system waits in five second increments at step 326until it has. If the system is a two-stage type, at block 324 thecontroller also monitors whether or not the second stage is currentlyactivated. If the second stage is activated, a recovery cycle will beinappropriate, and the system will again wait in five second incrementsrepresented by block 326 until the second stage is off. It can beappreciated that all of these steps may be essentially continuouslyperformed, with the microprocessor continuously monitoring the status ofthe pending call, temperatures, and timer, and waiting until allrequired conditions are fulfilled before moving to the next step.

[0083] Once the timer has expired, the controller initiates energyrecovery at step 328, by, for example, opening relay 138 if theapparatus of FIG. 4 is used to implement this control procedure.Following the initiation of recovery at step 328, the short cycle timeris set and started at step 330. Once recovery is initiated and thecompressor 66 is off, the air in the supply duct begins to warm, and tolose heat to the cold ducting material, other mechanical components ofthe HVAC system, and structural elements of the controlled space. Energyrecovery thus continues as the supply air temperature warms, and thecontroller will wait until certain conditions are met beforere-initiating cooling.

[0084] At this stage of the procedure, the short cycle timer, which wasstarted at step 330, is checked at step 332 to see if it is timed out.If not, the controller then waits, in five second increments illustratedby step 334, for the short cycle timer to time out. The short cycletimer thus ensures an off time which may advantageously be programmed tohelp ensure that compressor operation is within the manufacturer'sspecifications.

[0085] As shown on FIG. 7C, once the short cycle timer times out, atstep 336 the system check to see if the approximate temperature of thesupply air is within 0.5 degrees of the approximate temperature of thecontrolled space (as may be determined by monitoring the air temperaturein the return duct). If it is, this indicates that supply and returntemperatures are equilibrating, and that therefore significant energyrecovery from system components is no longer occurring. As illustratedby step 338, the system monitors the call status to see if the coolingcall has been satisfied. If the cooling call has been satisfied, thesystem moves back to the start of the procedure at block 300 of FIG. 7A,and waits for the next call for cooling.

[0086] If the cooling call has not been satisfied, the system will thenloop back to block 310 to begin the next on-cycle of the airconditioner. As before, after the system repeats the initiation ofcooling at block 310, the minimum run timer is again set and started,and a new target temperature is calculated using the current airtemperature of the controlled space as a new base point.

[0087] Returning now to step 336 of FIG. 7C, if the supply temperatureT_(s) is not within 0.5 degrees of the return temperature T_(r), thesystem checks, at block 340, whether or not the current rate of changeof the supply temperature is less than 10% of the maximum rate of changedetected during the present off-cycle. In other words, is the slope ofthe supply temperature vs. time flattening out significantly, therebyindicating the onset of equilibration and reduction in the rate ofenergy recovery from system components. If the rate of change of thesupply temperature is still greater than 10% of the maximum obtainedduring the present off cycle, the system waits for 5 seconds at block342 and loops back to block 336 to re-compare the temperature of thereturn and supply air temperatures. Once this rate of change conditionis satisfied, the controller loops back to block 310 to re-initiatecooling assuming the call for cooling is still pending. However, if thepending call gets satisfied at some point during the recovery cycle, thecontroller loops back to block 300 to await the next call.

[0088] A system operating in accordance with the procedure of FIGS. 7Athrough 7C will therefore continue to operate an air conditioning unituntil a certain target temperature is reached and at least one minimumrun timer has expired. Energy recovery is then initiated, whichcontinues until the supply and return air temperatures are close to oneanother, or until the rate of change of the supply air temperatureflattens considerably. The system cycles between the on-state and therecovery state until the call for cooling is satisfied.

[0089] Another alternative control procedure for heating is illustratedin FIGS. 8A through 8C. This scheme is similar to that described withreference to FIGS. 7A through 7C. As with the procedure of FIG. 7, thesystem begins in FIG. 8A at step 350 monitoring whether or not a callfor heating is being made. If not, the system waits for a call at step352.

[0090] Once a call has been made, a heating minimum run timer is set andstarted. First, at step 358, a minimum run timer for the first heatingcycle is set and started. As in the FIG. 7 embodiment described above, aminimum run timer of a different duration may be set and started at step360 when the system loops back from a recovery cycle. Also in analogywith the FIG. 7 embodiment above, the first heating minimum run timermay be set longer than the minimum run timer which is effective forsubsequent furnace-on cycles.

[0091] At step 362, the controller computes a target temperature for theair returning from the controlled space. This temperature is computed toensure that the controlled space temperature is increased by a minimumamount prior to the initiation of energy recovery. In some embodimentsof the present invention, the target temperature may be calculated withthe current return air temperature, a user programmable minimum heatingrate (which may advantageously be set to 2 to 5 degrees F. per hour) andthe initial set value of the minimum run timer, which may advantageouslyalso be user programmable. In one embodiment, the target temperature iscalculated by calculating the temperature increase produced by a heatingof the air produced by maintaining the user programmed minimum rate forthe duration of the minimum run timer initial setting. For example, ifthe programmable minimum heating rate is 3 degrees per hour, and theminimum run timer is set to 6 minutes, the target temperature is set to3 degrees per hour times 0.1 hours, or 0.3 degrees warmer than thecurrent return air temperature.

[0092] At step 364, heating is initiated. As discussed above withreference to FIGS. 5 and 6, this will occur immediately upon receipt ofthe call if the apparatus implementing this procedure is made inaccordance with FIG. 4.

[0093] While the furnace is activated, at block 366 the system measuresthe temperature of the air in the controlled space. As mentioned above,this measurement can be made by directly sensing temperature in thecontrolled space, or by sensing the temperature in a return air duct. Ifthe return temperature has not warmed to the target temperature, thecontroller performs no further action and at block 368 waits fiveseconds before making another measurement at block 366. Thus, thecontroller remains in the loop defined by blocks 366 and 368 until theapproximate temperature of the air in the controlled space increasesabove the target temperature.

[0094] Referring now to FIG. 8B, once the approximate temperature of thecontrolled space rises above the target temperature, the system checksif the heating call is satisfied at step 370. If it is, the systemreturns to block 350 on FIG. 8A, and waits for the next heating call. Ifthe heating call has not been satisfied, at step 372 the controllerchecks the status of the minimum run timer which was set and started atstep 358 (or step 360 if this is not the first cycle following a callfor heating) described above. As long as this timer has not expired, thesystem waits in five second increments at step 376 until it has. If thesystem is a two-stage type, at block 374 the controller also monitorswhether or not the second stage is currently activated. If the secondstage is activated, a recovery cycle will be inappropriate, and thesystem will again wait in five second increments represented by block376 until the second stage is off. As discussed above, it can beappreciated that all of these steps may be essentially continuouslyperformed, with the microprocessor continuously monitoring the status ofthe pending call, temperatures, and timer, and waiting until allrequired conditions are fulfilled before moving to the next step.

[0095] Once the timer has expired, the controller initiates energyrecovery at step 378, by, for example, opening the relay contacts of therelay 136 if the apparatus of FIG. 4 is used to implement this controlprocedure. Following the initiation of recovery at step 378, the shortcycle timer is set and started at step 380. Once recovery is initiatedand the furnace is off, the air in the supply duct begins to cool, andto remove heat from the warm ducting material, other mechanicalcomponents of the HVAC system, and structural elements of the controlledspace. Energy recovery thus continues as the supply air temperaturecools toward the ambient temperature, and the controller will wait untilcertain conditions are met before re-initiating heating.

[0096] At this stage of the procedure, the short cycle timer, which wasstarted at step 380, is checked at step 382 to see if it is timed out.If not, the controller then waits, in five second increments illustratedby step 384, for the short cycle timer to time out. The short cycletimer thus ensures an off time which may advantageously be programmed tohelp ensure that furnace operation is within the manufacturer'sspecifications.

[0097] Referring now to FIG. 8C, once the short cycle timer times out,at step 386 the system check to see if the approximate temperature ofthe supply air is within 0.5 degrees of the approximate temperature ofthe controlled space (as may be determined by monitoring the airtemperature in the return duct). If it is, this indicates that supplyand return temperatures are equilibrating, and that thereforesignificant energy recovery from system components is no longeroccurring. As illustrated by step 388, the system monitors the callstatus to see if the heating call has been satisfied at block 388. Ifthe heating call has been satisfied, the system moves back to the startof the procedure at block 350 of FIG. 8A, and waits for the next callfor heating.

[0098] If the heating call has not been satisfied, the system will thenloop back to block 358 to begin the next on-cycle of the furnace. Asbefore, after the system repeats the initiation of heating at block 360,the minimum run timer is again set and started, and a target temperatureis calculated using the current air temperature of the controlled spaceas a new base point. It may also be noted that for this and subsequenton-cycles, the minimum first run timer is not re-set or re-started.Thus, the system effectively waits only for the attainment of the targettemperature and the expiration of the minimum run timer at block 374before initiating another energy recovery cycle at block 378 of FIG. 8B.

[0099] Returning now to step 386 of FIG. 8C, if the supply temperatureT_(s) is not within 0.5 degrees of the return temperature T_(r), thesystem checks, at block 390, whether or not the current rate of changeof the supply temperature is less than 10% of the maximum rate of changedetected during the present off-cycle. In other words, is the slope ofthe supply temperature vs. time flattening out significantly, therebyindicating the onset of equilibration and reduction in the rate ofenergy recovery from system components. If the rate of change of thesupply temperature is still greater than 10% of the maximum obtainedduring the present off cycle, the system waits for 5 seconds at block392 and loops back to block 386 to re-compare the temperature of thereturn and supply air temperatures. Once this rate of change conditionis satisfied, the controller loops back to block 360 to re-initiateheating assuming the call for heating is still pending. However, if thepending call gets satisfied at some point during the recovery cycle, thecontroller loops back to block 350 to await the next call.

[0100] Thus, in analogy with FIGS. 7A through 7C, a system operating inaccordance with the procedure of FIGS. 8A through 8C will continue tooperate a furnace until a certain target temperature is reached and atleast one minimum run timer has expired. Energy recovery is theninitiated, which continues until the supply and return air temperaturesare close to one another, or until the rate of change of the supply airtemperature flattens considerably. The system cycles between theon-state and the recovery state until the call for heating is satisfied.

[0101] It can be appreciated that in many embodiments of the abovedescribed procedures, the various evaluation of environmental andphysical parameters such as temperature, humidity, durations, etc. willbe essentially constantly performed. For example, when implementingthese methods with the controller of FIG. 4, the humidity can beconstantly monitored, and the controller can be disabled from affectingthe normally closed state of the relays 136, 138 until the humiditydrops below its setpoint. Thus, the particular order and sequence of theflowcharts of FIGS. 5 through 8 is not intended to indicate that thisorder is required. In some embodiments of these methods implemented withthe apparatus of FIGS. 3 and 4 for example, it can be seen that thecalls for cooling and heating still directly control the furnace andcompressor, and thus the controller cannot force heating or cooling tooccur when no call is present. Thus, the initiation of heating orcooling can be implemented by simply allowing the normal cooling orheating calls to pass through the controller. Many different specificimplementations of parameter monitoring and HVAC control in accordancewith principles of the present invention will be possible to those ofordinary skill in the art based on this disclosure.

[0102] The graphs of FIGS. 9 through 12 illustrate energy consumptionlevels and energy output levels for a typical five ton HVAC systemresponding to an example 30 minute call for environmental modificationwith an energy controller operating in accordance with some principlesof the present invention. FIG. 9 illustrates BTU per minute of gas useover the 30 minute period. Drop offs 400, 402, 404 in gas use indicatethe initiation of energy recovery. Increases in gas use 406, 408,indicate re-initiation of heating after each period of energy recovery.When the gas is on, an energy equivalent of 4,444 BTUs per minute arebeing consumed in the furnace. The average energy consumption over theseveral on/off cycles of gas use is 2,645 BTUs.

[0103]FIG. 10 is a graph of BTU output to the controlled space duringthe period of gas use shown in FIG. 9. The instantaneous BTU output 410has a peak of approximately 3400 BTU per minute delivered to thecontrolled space. The average BTU output 412 to the room isapproximately 2455 BTU per minute. Due to energy recovery from systemcomponents, the decrease in average energy input is greater than thedecrease in average energy output. An increase in heating efficiency isthus attained.

[0104]FIGS. 11 and 12 demonstrate a similar effect in a cooling mode.The compressor is effectively cycled, resulting in energy consumptionreductions at points 416 and 418 of approximately 80%. The remaining 20%of the power consumed during energy recover periods is consumed mainlyby the air supply fan, which as described above, preferably remainsoperational. Referring now to FIG. 12, it can be seen that in a manneranalogous to the heating graph of FIG. 10, the instantaneous BTU perminute removed the controlled space in cooling 420 rises and fallsdepending on whether or not the HVAC controller is in energy recoverymode. The average energy transfer 422 however, remains high enough toproduce a significant increase in cooling efficiency.

[0105] The above described invention therefore provides many advantagesover prior art HVAC control systems and methods. One major benefit isthe provision of reduced energy consumption without significantlyreducing the comfort of occupants of the controlled space. In addition,safeguards such as minimum run timers and short cycle timers may beprovided to protect the HVAC equipment form over-cycling.

[0106] The digital nature of the preferred embodiment also allows for aconvenient programmable mode of operation which allows energy savings tobe determined empirically with a high degree of accuracy. The HVACcontroller of the present invention can be programmed to refrain fromentering energy recovery mode on alternating 24 hour periods. Relevantdata such as on and off times and energy outputs may be stored in theon-board memory over an extended 30 to 60 day test period. The totalenergy consumption of the environmental control system is compared forthe two periods, one with the energy recovery mode operational, and theother without. The energy consumption of the system can be estimated bymeasuring on-time durations for heating and cooling, or can be measuredmore directly by gas flow sensors and ammeters or watt meters situatedto provide essentially direct power consumption measurements.Furthermore, variations in the duration of recovery disabled mode vs.recovery enabled mode can be made depending on the nature of thespecific installation. Advantageously, the data can be made available tosystem administrators via the I/O port.

[0107] The foregoing description details certain preferred embodimentsof the present invention and describes the best mode contemplated. Itwill be appreciated, however, that no matter how detailed the foregoingappears in text, the invention can be practiced in many ways. As is alsostated above, it should be noted that the use of particular terminologywhen describing certain features or aspects of the present inventionshould not be taken to imply that the broadest reasonable meaning ofsuch terminology is not intended, or that the terminology is beingre-defined herein to be restricted to including any specificcharacteristics of the features or aspects of the invention with whichthat terminology is associated. The scope of the present inventionshould therefore be construed in accordance with the appended Claims andany equivalents thereof.

What is claimed is:
 1. A method of controlling an HVAC systemcomprising: receiving a call for heating; turning on a furnace;monitoring the on-time of said furnace; sensing a controlled spacetemperature; comparing said controlled space temperature with apredetermined value; and shutting off said furnace when (1) said on-timereaches a predetermined minimum, and (2) said controlled spacetemperature is greater than said predetermined value.
 2. The method ofclaim 1, wherein said predetermined value substantially minimizesoccupant discomfort.
 3. The method of claim 2, wherein saidpredetermined value is approximately 55 degrees fahrenheit.
 4. Themethod of claim 1, additionally comprising turning said furnace back onin response to a decrease in the temperature of said controlled space.